fix(auth): a tab acts only as the account it was built for, and the review's other findings

A tab names the account it was claimed for on every request, beacon and upload
(X-Siftlode-Expect-Account); current_user and /auth/logout refuse a mismatch with
401 account_changed and keep the session, so a tab left showing the account
signed in before reloads instead of writing to the new one.

/auth/login clears a connect marker left by an abandoned connect, and a link
completes only on a session that still holds. The prefs merge reads the row
under a lock. A tab's sessionStorage records its own owner. A page the account
loses is given up, address bar and remembered page included. The panel layouts
are preferences; the storage contract also refuses a key handed to the Storage
API or to usePref outside its kind. A refused sign-in as someone else and a
failed sign-out say so. The chromium-only e2e specs are in the gate register,
the member walk waits for its requests, and devices.spec checks the save itself.

frontend/src/lib/staleBuild.ts

@@ -8,8 +8,9 @@
 // minute at most: a chunk that is missing from the NEW build too would otherwise reload forever, so
 // the second failure inside the minute is let through to the error boundary, which says so.
 import { log } from "./log";
+import { LS } from "./storage";
 
-const KEY = "sf.staleBuildReload";
+const KEY = LS.staleBuildReload;
 const WINDOW_MS = 60_000;
 
 let reloading = false;